The present invention relates to liquid crystal displays and more particularly to systems for use in correcting for image retention and flicker problems exhibited by typical active matrix liquid crystal displays.
In most active matrix liquid crystal displays the data signals applied to the individual liquid crystal pixels are subject to distortion resulting from the gate-source capacitances which are characteristic of the thin filled transistors used in driving such pixels and the capacitances exhibited by the pixels themselves. As shown in FIG. 1, gate drive pulses 10 of amplitude V.sub.G are periodically applied to the scanning or select lines of a display matrix in order to enable data signals 12 of either positive or negative polarity to be applied to the pixel electrodes of the liquid crystal pixels in the matrix. However, the gate-source capacitances of the thin film transistors driving the pixels affect the waveform of the pixel drive signal 14 as charge is diverted at the falling edges of the gate drive pulses to satisfy the capacitance requirements of the gate-source junctions of the thin film transistors resulting in a voltage distortion .DELTA.V in the voltage level at the pixel electrodes. The voltage distortion .DELTA.V constitutes a DC offset having longer term effects on the liquid crystal pixels and resulting in significantly degraded image quality due to image retention and flicker.
In accordance with past practices for correcting this problem small DC bias voltages have been applied across liquid crystal display matrixes in an attempt to compensate for the voltage distortion .DELTA.V. However, this technique has not proven entirely satisfactory since the amount of voltage distortion exhibited by each pixel is a function of the construction of the individual liquid crystal pixels and more importantly is a non-linear function of the data signal voltage level. Consequently, in active matrix liquid crystal displays in which data voltages are controlled to provide a gray scale for use in furnishing enhanced images, the bias voltage to be applied across the matrix for compensating for the voltage distortion due to the inherent parasitic capacitances can only be approximated to an average level resulting in continued image retention and flicker problems.
It is therefore an object of the present invention to provide a system for eliminating image retention and flicker problems in active matrix liquid crystal displays.
It is another object of the present invention to provide a construction for a liquid crystal display matrix including devices and methods for accurately compensating on a pixel-by-pixel basis for the parasitic gate-source capacitances of the thin film transistors used in driving the liquid crystal pixels in the matrix.
It is a further object of the present invention to provide a system for suppressing image retention and flicker problems in active matrix liquid crystal displays which is simple in operation and can be readily implemented into a liquid crystal matrix designs at minimum expense and with a minimum of effort.